In a cell phone, the condition can exist where a fully (or nearly so) charged battery is simultaneously connected to both the output of the battery charger and the system load (e.g., RF transmitter, etc.). In this case, the battery will spend most of the time at the charger float voltage. The float voltage will normally be close to the maximum allowable battery voltage established by the battery manufacturer and/or safety standards organizations. If the system suddenly demands a lot of current, the required current will initially be supplied by the battery, due to the limited bandwidth of the charger, and the battery voltage will droop. This droop is not harmful, but when the system load current demand is removed, the battery voltage may overshoot, resulting in a transient battery overvoltage condition. In particular, the TDMA protocol employed in GSM cell phones produces a GPRS pulse train, consisting of (typically) 2 A for 1.15 ms and 0 A for 3.45 ms. Existing battery charger ICs all suffer from significant battery voltage overshoot in response to a GPRS pulse.
In Japan, JEITA specifications provide an example of the motivation for eliminating the overshoot: they prohibit repetitive excursions over 4.40 V for lithium-ion batteries. With tolerance stack-up, overshoot causes customers to program their float voltages lower than they could if there was no overshoot. This margining due to overshoot reduces the amount of charge put into the battery, thereby reducing talk time by as much as 10%.
Although the following Detailed Description will proceed with reference being made to illustrative embodiments, many alternatives, modifications, and variations thereof will be apparent to those skilled in the art.